The present invention relates to a reference voltage generation circuit for driving a group of driven elements such as, for example, an array of light emitting diodes (LEDs) disposed in an electro-photography printer as a light source, an array of heating resistors disposed in a thermal printer, and an array of display units disposed in a display device. The present invention also relates to a drive circuit including the reference voltage generation circuit; a light emitting diode (LED) head including the drive circuit; and an image forming apparatus including the light emitting diode (LED) head.
In the specification, a light emitting diode may be referred to as an LED; a monolithic integrated circuit may be referred to as an IC (Integrated Circuit); an n-channel MOS (Metal Oxide Semiconductor) transistor may be referred to as an NMOS; and a p-channel MOS transistor may be referred to as a PMOS.
Further, a signal terminal and a signal input to or output from the signal terminal may be designated with a same reference designation. A static latent image formed on a photosensitive drum according to each of light emitting elements, or a toner image after development or transferred to a printing medium may be referred to as a dot. Each of the light emitting elements corresponding to the dot also may be referred to as a dot.
An LED head is a generic nomenclature of a unit in which a light emitting element and a drive element thereof are disposed. When the LED head is disposed only in a printer device, the LED head is referred to as an LED print head. In the following description, a group of driven elements is an array of LEDs used in an electro-photography printer as an example.
In a conventional image forming apparatus such as an electro-photography printer, a photosensitive drum charged is selectively irradiated according to print information, thereby forming a static latent image thereon. Then, toner is attached to the static latent image to form a toner image. Afterward, the toner image is transferred to a sheet, so that the toner image is developed. An LED is used as a light source. An LED head used in the conventional printer is formed of an LED array chip having a plurality of LED elements and a driver IC for driving the LED array chip.
The LED head includes a reference voltage generation circuit for generating a reference voltage, so that a drive current for driving the LED elements is determined based on the reference voltage generated from the reference voltage generation circuit and a resistor disposed in the driver IC. The resistor is produced through a semiconductor process technology. In general, the resistor is formed of poly-silicon or an impurity diffused resistor, and is integrated in the driver IC in a form of monolithic.
In the conventional electro-photography printer, the LED elements have light emission power having temperature dependence with a negative temperature coefficient. Accordingly, when a junction temperature of the LED array chip increases, the light emission power decreases.
For example, when the LED is formed of a GaAsP element, the temperature coefficient is about −0.6%/° C. When the LED is formed of an AlGaAs element, the temperature coefficient is −0.25%/° C. When the LED is formed of a GaAs element, the temperature coefficient is −1.0%/° C. Depending on a composition of a semiconductor compound or a luminescence wavelength, the temperature coefficient of the light emission power varies significantly.
As described above, the driver IC of the LED elements is disposed in the LED head. Accordingly, it is preferable that the temperature coefficient of the LED drive current value becomes positive, thereby compensating the negative temperature coefficient of the LED light emission power. The LED drive current value is determined based on the resistor disposed in the IC driver and the value of the voltage output from the reference voltage generation circuit. Accordingly, considering a temperature coefficient of the resistor (generally positive value), it is necessary to provide the voltage output from the reference voltage generation circuit with a positive temperature coefficient.
As described above, even when the temperature varies upon the LED drive, it is necessary to maintain the light emission power at a specific level. To this end, it is necessary to provide a drive method for compensating the temperature dependence of the light emission power of the LED elements. Patent Reference 1 has disclosed a circuit having such a temperature compensation circuit as explained below.    Patent Reference 1: Japanese Patent Publication No. 10-332494
FIG. 20 is a circuit diagram showing a drive circuit of the LED head of the conventional printer. FIG. 21 is a circuit diagram showing a conventional reference voltage generation circuit 37′ disclosed in Patent Reference. More specifically, FIG. 20 is a circuit diagram showing a main portion of the driver IC. FIG. 20 shows a connection relationship between the LED drive circuit and a peripheral circuit thereof, and one LED element (one dot) is shown in FIG. 20.
As shown in FIG. 20, the LED drive circuit includes a pre-buffer circuit G1′ indicated with a hidden line, and the pre-buffer circuit G1′ is formed of an AND circuit 42′, a PMOS transistor 43′, and an NMOS transistor 44′. Further, the LED drive circuit includes an inverter circuit G0′, a latch circuit LT1, and a control voltage generation circuit 36′ indicated with a projected line. The control voltage generation circuit 36′ is disposed per one driver IC chip.
An operational amplifier 51′ outputs a voltage Vcont (control potential) to an LED drive transistor Tr1′ for adjusting a drive current of an LED element LD1′. Further, the LED drive circuit includes a resistor 53′ having a resistivity of Rref, and a PMOS transistor 52′ having a gate length the same as that of the LED drive transistor Tr1′.
A reference voltage input terminal VREF is connected to an reverse input terminal of the operational amplifier 51′, so that a reference voltage Vref generated at the reference voltage generation circuit (described later) is input. The operational amplifier 51′, the PMOS transistor 52′, and the resistor 53′ constitute a feedback control circuit. A current Iref flowing through the resistor 53′, that is, the PMOS transistor 52′, is not depended on a power source voltage VDD, and is determined only by the reference voltage Vref and the resistivity Rref of the resistor 53′.
The operational amplifier 51′ controls such that a potential of the reverse input terminal thereof becomes equal to a potential of a non-reverse input terminal thereof. Accordingly, the current Iref flowing through the resistor 53′ is given by:Iref=Vref/Rref
As described above, it is configured such that the PMOS transistor 52′ has the gate length the same as that of the LED drive transistor Tr1′. A gate potential thereof becomes equal to the voltage Vcont upon driving the LED element. Accordingly, the PMOS transistor 52′ and the LED drive transistor Tr1′ operate in a saturated region, and have a current-mirror relationship.
As a result, a drive current value of the LED element LD1′ is proportional to the current Iref flowing through the resistor 53′, and the current Iref is proportional to the reference voltage Vref input into the VREF terminal. Accordingly, it is possible to collectively adjust the LED drive current according to the reference voltage Vref.
FIG. 21 is a circuit diagram showing the conventional reference voltage generation circuit 37 for generating the reference voltage Vref.
As shown in FIG. 21, PMOS transistor 61′, 62′, and 63′ with a same size have source terminals connected to the power source VDD and gate terminals connected to each other, thereby constituting a current-mirror circuit. A drain terminal of the PMOS transistor 61′ is connected to a collector terminal of an NPN bipolar transistor 64′ through resistor 66′ and 67′ connected in series. The NPN bipolar transistor 64′ has an emitter terminal connected to ground and a base terminal connected to a connection point of the resistors 66′ and 67′.
A drain terminal of the PMOS transistor 62′ of the current-mirror circuit is connected to a collector terminal of an NPN bipolar transistor 65′. The NPN bipolar transistor 65′ has an emitter terminal connected to ground and a base terminal connected to the collector terminal of the NPN bipolar transistor 64′. A drain terminal of the PMOS transistor 63′ is connected to ground through a resistor 68′.
The NPN bipolar transistor 65 has an emitter area N times larger than an emitter area of the NPN bipolar transistor 64′ (N>1). A connection point of the drain terminal of the PMOS transistor 63′ and the resistor 68′ becomes an output terminal of the conventional reference voltage generation circuit 37′ for outputting the reference voltage Vref.
As disclosed in Patent Reference 1, the conventional reference voltage generation circuit 37′ shown in FIG. 21 generates an output voltage having a positive temperature coefficient. As shown in FIG. 21, the resistors 66′, 67′, and 68′ have resistivities of R1, R2, and R3, respectively.
In the conventional reference voltage generation circuit 37′ shown in FIG. 21, it is assumed that a base current of the bipolar transistors 64′ and 65′ is negligibly small relative to a collector current thereof. Accordingly, the output voltage Vref of the conventional reference voltage generation circuit 37′ is given by:Vref=(R3/R2)×(kT/q)ln(N)Where k is the Boltzmann constant, T is an absolute temperature, q is a charge of electron, and ln represents natural legalism.
It is supposed that a temperature coefficient Tc of the output voltage Vref is defined by:Tc=(1/Vref)×(ΔVref/ΔT)
Accordingly, the temperature coefficient Tc of the output voltage Vref is given by 1/T, and becomes about +0.33%/° C. at a room temperature (about 300° K.).
In the LED element formed of a GaAlAs element and used in the LED head, the temperature dependence of the light emission power is about −0.25%/° C. A temperature dependence of a reference resistor disposed in the IC driver formed through a CMOS process is about +0.1%/° C.
A temperature of the LED element is about the same as a temperature of the IC driver arranged adjacent to the LED element. Further, the LED elements and the conventional reference voltage generation circuit 37′ may be arranged on a ground wiring portion formed on a print circuit board, so that each of the LED elements has a similar temperature.
Accordingly, in order to compensate the reduction in the light emission power upon an increase in the temperature of the LED elements, it is suffice that the reference voltage Vref has the following temperature coefficient:−(−0.25−0.1)=+0.35%/° C.
The temperature coefficient thus obtained is about the same as the temperature coefficient of the conventional reference voltage generation circuit 37′.
Patent Reference 2 has disclosed another conventional reference voltage generation circuit 38′. FIG. 22 is a circuit diagram showing the conventional reference voltage generation circuit 38′.    Patent Reference 2: Japanese Patent Publication No. 2006-159472
As shown in FIG. 22, the conventional reference voltage generation circuit 38 includes a regulator circuit 71′, diodes 72′ and 73′, and resistors 74′ and 75′. A first terminal of the regulator circuit 71′ is a power source terminal connected to the power source VDD. Further, a second terminal of the regulator circuit 71′ is an output terminal connected to an anode terminal of the diode 72′, and a third terminal of the regulator circuit 71′ is a ground terminal connected to ground.
A cathode terminal of the diode 72′ is connected to an anode terminal of the diode 73′. A cathode terminal of the diode 73′ is connected to one end portion of the resistor 74′. The other end portion of the resistor 74′ is connected to ground through the resistor 75′. A middle connection point of the resistors 74′ and 75′ is connected to a reference voltage terminal Vref.
In the conventional reference voltage generation circuit 38 shown in FIG. 22, when the regulator circuit 71′ has an output voltage Vo; the diodes 72′ and 73′ have a forward voltage Vf; and the resistors 74′ and 75′ have resistivities R1 and R2, a cathode voltage Vk of the diode 73′ is given by:Vk=Vo−2×Vf 
Further, the reference voltage Vref is given by:Vref=R2×Vk/(R1+R2)=R2×(Vo−2×Vf)/(R1+R2)
In the above equations, the forward voltage Vf of the diodes 72′ and 73′ decreases at a rate of −2 mV/° C. with an increase in a temperature. Accordingly, the reference voltage Vref increases with an increase in a temperature substantially linearly.
The resistors 74′ and 75′ have small temperature dependence, and the regulator circuit 71′ also has small temperature dependence, thereby making it possible to ignore the temperature dependence thereof. Accordingly, in the conventional reference voltage generation circuit 38′ shown in FIG. 22, a temperature coefficient Tc of the reference voltage Vref is given by:Tc=1/Vref×(ΔVref/ΔVDD)=2×(Vo−2×Vf)/(−ΔVf/ΔT)
As a first example, when the forward voltage Vf of the diodes 72′ and 73′ is 0.6 V, a temperature coefficient of the forward voltage Vf is −2 mV/° C., and the output voltage Vo of the regulator circuit 71′ is 2.5 V, the temperature coefficient Tc of the reference voltage Vref is given by:Tc=1/Vref×(ΔVref/ΔVDD)=2×(2.5−2×0.6)/(2 mV/° C.)=+0.31%/° C.
Further, the reference voltage Vref is given by:Vref=2.5−2×0.6=1.3 V
As a second example, when the output voltage Vo of the regulator circuit 71′ is 1.87 V, the reference voltage Vref is given by:Vref=1.87−2×0.6=0.67 V
Further, the temperature coefficient Tc of the reference voltage Vref is given by:Tc=2×(2 mV/° C.)/0.67=0.6%/° C.
Accordingly, the temperature coefficient Tc in the second example becomes double of that in the first example. However, the reference voltage Vref in the second example becomes half of that in the first example.
As described above, the temperature dependence of the LED drive IC is about −0.1%/° C. Combining with the conventional reference voltage generation circuit 38′, the temperature coefficient of the LED drive current becomes about +0.5%/° C., thereby being suitable for compensating a temperature of the LED elements with a temperature dependence of about −0.5%/° C.
As a third example, when the output voltage Vo of the regulator circuit 71′ is 1.56 V, the reference voltage Vref is given by:Vref=1.56−2×0.6=0.36 V
Further, the temperature coefficient Tc of the reference voltage Vref is given by:Tc=2×(2 mV/° C.)/0.36=1.1%/° C.
Accordingly, the temperature coefficient Tc in the third example becomes double of that in the second example. However, the reference voltage Vref in the third example becomes half of that in the second example.
As described above, the temperature dependence of the LED drive IC is about −0.1%/° C. Combining with the conventional reference voltage generation circuit 38′, the temperature coefficient of the LED drive current becomes about +1.0%/° C., thereby being suitable for compensating a temperature of the LED elements with a temperature dependence of about −1.0%/° C.
In the LED head, even though the temperature varies associated with the LED rive, it is necessary to maintain the light emission power at a specific level. Accordingly, it is necessary to provide a driving method capable of compensating the temperature dependence of the light emission power of the LED elements. The LED elements tend to have various temperature dependences. Accordingly, it is necessary to provide a temperature compensation circuit with a simple configuration for obtaining a specific temperature coefficient.
In the conventional reference voltage generation circuit 37′ shown in FIG. 21, it is possible to obtain only the specific temperature coefficient disproportional to the absolute temperature. Accordingly, it is difficult to use the conventional reference voltage generation circuit 37′ for compensating a temperature of the LED elements with various temperature dependences.
In the conventional reference voltage generation circuit 38 shown in FIG. 22, it is possible in principle to compensate a temperature of the LED elements with temperature dependences of −0.5%/° C. and −1.0%/° C. However, when the temperature coefficient increases, the reference voltage Vref decreases to 0.67 V or 0.36 V, thereby making it difficult to obtain a desirable reference voltage for the driver IC.
Alternatively, it is possible to increase a connection stage of the diodes in the series connection to three or four stages from the two stages in the conventional reference voltage generation circuit 38′ shown in FIG. 22. In this case, it is necessary to increase the number of the diodes, thereby increasing cost.
Further, it is also possible to increase the output voltage Vo of the regulator circuit 71′. However, when the output voltage Vo of the regulator circuit 71′ becomes close to the power source voltage of the power source VDD, it is difficult to obtain a desired property of the regulator circuit 71′. Further, when the output voltage Vo of the regulator circuit 71′ exceeds the power source voltage of the power source VDD, it is not practical.
In view of the problems described above, an object of the present invention is to provide a reference voltage generation circuit for compensating negative temperature dependence of light emitting power of an LED element and temperature dependence of a reference resistor in a driver IC. In the reference voltage generation circuit, it is possible to set an arbitrary temperature coefficient and an arbitrary output voltage. Another object of the present invention is to provide a drive circuit, a print head, and an image forming apparatus having the reference voltage generation circuit.
Further objects and advantages of the invention will be apparent from the following description of the invention.